In food shops in convenience stores, department stores, super markets, etc., foods, such as daily dishes, noodles, and salads are packaged in a food container, such as a tray, cup or bowl, and sold. The container is composed of a container body wherein a food is placed and a cover which seals the container body. The container body is, in general, manufactured by heat-forming a sheet of polypropylene, foamed polypropylene, polypropylene containing filler, polyethylene, foamed polyethylene, foamed polystyrene, heat-resistant foamed polystyrene, A-PET (amorphous PET) or the like, using a vacuum forming, pressure forming, or vacuum-pressure forming machine. The cover is formed of a sheet of A-PET, biaxially stretched polystyrene (OPS), polypropylene (PP) or the like (see Patent Document 1).
Recently, since uniaxially stretched PET film is highly transparent and heat-resistant, it is used for a touch panel and a liquid crystal display element relating to IT (see Patent Document 2, 3, 4).
Food purchased from a convenience store or the like is heated in the food container by a microwave oven. When a food containing oil is heated together with the food container by a microwave oven, the temperature of the food is elevated to about 150° C. Therefore, the food container is required to have a high heat resistance capable of resisting up to 150° C. Even, in the case of a food not containing oil, since the temperature is raised to around 90° C., the heat resistance resisting up to 100° C. is required.
Furthermore, high transparency is required for food containers, in order to recognize the contained food clearly at a glance and to improve commodity value.
However, in the aforementioned various sheets used heretofore, there is no sheet satisfying both high heat resistance and high transparency simultaneously. Namely, among these various sheets, although A-PET sheet and OPS (biaxially stretched polystyrene) have high transparency, these sheets are softened at about 80° C. and do not have high heat resistance. Although a PP sheet has high heat resistance, it is inferior in transparency.
Uniaxially stretched PET film used for a touch panel or the like has high transparency and heat resistance, but, as described in Patent Document 2, after being uniaxially stretched in TD (lateral), it is heat-set at 220° C. As a result, it is not elongated even by heating, and it is impossible to form into a container shape by a heat forming machine.
Thereupon, the applicant proposed a heat-resistant transparent container produced by primarily uniaxial stretching of an A-PET sheet at a stretching temperature of 90-120° C. at a draw ratio of 2.6-3.7 times in the MD (longitudinal direction), followed by heat-setting at a temperature higher than the stretching temperature by 5-20° C. in the primary stretching and heat-setting process, and subsequently, thermoforming the primarily stretched A-PET sheet at 80-130° C. while stretching secondarily, followed by secondary heat-setting in the same mold at 160° C. or more in the secondary stretching and heat-setting process, wherein the crystallinity of the primarily stretched A-PET through the primary stretching and heat-setting process is 22% or more and less than 30% represented by the following formula, and the crystallinity of the container cooled through the aforementioned secondary stretching and heat-setting process is 30% or more represented by the following formula (see Patent Document 5).
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